Dealkylation of alkyl aromatics using a hydrogenated aromatic hydrogen donor



United States Patent 3,485,883 DEALKYLATION 0F ALKYL AROMATICS USING A HYDROGENATED AROMATIC HYDROGEN DONOR Robert M. Engelbrecht, St. Louis, James C. Hill, Chesterfield, and Richard N. Moore, St. Louis, Mo., assi'gnors to Monsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed Dec. 27, 1966, Ser. No. 604,614 Int. Cl. C07c 3/58 U.S. Cl. 260-672 7 Claims ABSTRACT OF THE DISCLOSURE Alkyl aromatic hydrocarbons are thermally dealkylated in the presence of an aromatic hydrocarbon hydrogen donor which has been at least partially hydrogenated. For example, with pure toluene as the alkylaromatic, demethylation occurred at about four times the rate (12.5 vs 3.2% conversion to benzene) when fed along with 1:1 mol ratio of 9,10-dihydrophenanthrene as a hydrogen source instead of molecular hydrogen for thirteen seconds residence time at 1157 F. and 48.6 atmospheres pressure.

This application relates to assignees copending applications Ser. No. 527,049 and Ser. No. 603,089.

The present invention relates to a process for the conversion of alkyl aromatic compounds. More particularly, the present invention relates to a process for the non-catalytic dealkylation of alkyl substituted aromatic compounds.

Thermal dealkylation of alkyl aromatic compounds as a means of increasing the value of fracti ns rich in such compounds has within recent years gained significant importance. Processes for the thermal dealkylation of alkyl substituted aromatic compounds generally involves subjecting such compounds to elevated temperatures and pressures in the presence of added hydrogen and in the absence of a catalyst. While thermal dealkylation has reached the stage of being an accepted commercially feasible means of increasing the value of alkyl aromatic containing fractions, there is still substantial room for improving such processes. As is usual with virtuall any conversion process, theer is a constant need to find means of increasing the efiiciency of the thermal dealkylation of alkyl substituted aromatic compounds by incr asing conversion rate and/ or increasing yields. Additionally, it is desirable to decrease residence times in order to bring about a greater production of dealkylated product per unit time.

It is an object of the present invention to provide a process for the dealkylation of alkyl substituted aromatic compounds. Another object of the present invention is to provide a process for the non-catalytic thermal dealkylation of alkyl substituted aromatic compounds. It is also an object of the present invention to provide a process whereby the alkyl substituents of alkyl substituted aromatic hydrocarbons may be removed without benefit of a catalyst and wherein a high rate of hydrogen use is obtained. Another object of the present invention is to provide a process for the dealkylation of alkyl substituted aromatic hydrocarbons whereby the alkyl side chains are cleaved from the aromatic ring with high efiiciency. Additional objects will become apparent from the following description of the invention herein disclosed.

The present invention which fulfills these and other objects, is a process which comprises subjecting a feed stock containing alkyl substituted aromatic compounds to a temperature of 1020 to 1475 F. and a pressure of Patented Dec. 23, 1969 to 3000 p.s.i.g. in a non-catalytic thermal reaction zone in the presence of a hydrogen donor, said hydrogen donor being an aromatic hydrocarbon which has been at least partially hydrogenated. The process of the present invention provides a method for the thermal dealkylation of alkyl aromatic compounds wherein high conversion rates of alkyl aromatic compound to dealkylated product are obtained. A further advantage of the present process is that the alkyl substituent-s generally are severed from the alkyl aromatic compound as one molecule rather than as several lower molecular weight molecules thereby preserving the alkyl molecule and reducing the hydrogen consumption.

To further describe and particularly to demonstrate the present invention, the following examples are presented. These examples are not to be construed in any manner as limiting the present invention.

EXAMPLE I A series of five dealkylation runs were carried out with toluene as the alkyl substituted aromatic hydrocarbon. In each of the runs, toluene was introduced into a reaction tube concurrently with 9,10-dihydrophenanthrone in a molar ratio of 1:1. The reaction tube was 7% inches in length with a diameter varying from inch at the entrance to /8 inch at the exit and was packed in the last 5% inches with inch diameter Alundum balls which were used to promote heat transfer and limit backmixing. The conditions of the five runs with respect to temperature, pressure and residence time and the percent conversion of toluene to benzene as well as the efficiency of hydrogen use, as a percentage, are given in the table below.

Contact Percent Hydrogen Use, Percent Tempera- Time, Pressure, Converture, F. Seconds p.s.i.g. sion Consumed Efficiency The hydrogen use percent consumed is calculated according to the following formula:

The hydrogen use percent efiiciency is calculated by the formula:

Mols hydrogen consumed Mols hydrogen available =Hydrogen use percent consumed Mols toluene fraction converted Mols hydrogen percent consumed =Hydrogen use percent efi'ieiency From the above table it should be noted that the temperature and contact time are relatively critical in the practice of the process of the present invention. At a contact time of 6 seconds almost a three fold increase in conversion was obtained by increasing the temperature from 1112 F. to 1202" F. At 13 seconds contact time a little better than 50% increase in conversion was obtained by going from 1112 F. to 1157 F.

EXAMPLE II Conver- Hydrogen Molar Flow, sion to Hydrogen Use, Percent Source Toluene Benzene, Toluene (Moles/Sec.) Percent Consumed Efficiency 1:1 0.613X10- 12. 5 16. 2 77 3. 6: 1 0.613X1O- 7. 2 2. 0 100 1:1 1.360X10- 3. 2 3. 4 Q4 The above Example II demonstrates the substantially improved rate of hydrogen use obtained by means of the present process. Also, it should be noted that while a decrease in hydrogen use efiiciency is obtained in accordance with the process of the present invention, there is also obtained a very substantial increase in the conversion rate of the-toluene to benzene by the present process.

EXAMPLE III Two runs are carried out substantially as described in Example I above with the exception that in both runs the temperature is 1250 F. and in one run l-ethyl naphthalene is the feed and in the other run, ethyl benzene is the feed. In each of the runs, 9,10-dihydrophenanthrene is used as the hydrogen donor in a molar ratio to the alkyl aromatic hydrocarbon feed of 1:1. The pressure within the reaction chamber is 700 p.s.i.g. Residence time in both runs is 13 seconds. In both runs, a high rate of hydrogen use is obtained along with a substantial conversion of the alkyl aromatic hydrocarbon to the corresponding aromatic ring compound.

The process of the present invention is equally applicable to the dealkylation of alkyl substituted mononuclear aromatic compounds and poly-nuclear aromatic compounds or mixtures thereof. For example, the present invention works equally well in the dealkylation of toluene, methyl naphthalenes, methyl phenanthrenes, methyl anthracenes, methyl chrysenes, and the like. Additionally, the process of the present invention may be employed for the dealkylation of alkyl substituted heterocyclic compounds such as alkyl pyridines, alkyl pyrans, alkyl furans, alkyl thiophenes and the like. The process of the present invention may be utilized in the dealkylation of alkyl aromatic compounds having any number of carbon atoms in the alkyl substituent. Particularly useful feeds to the process of the present invention include alkyl aromatic hydrocarbons such as the mono-, dior tri-alkyl substituted aromatic hydrocarbons. Several non-limiting examples of such aromatic hydrocarbons are toluene, methyl naphthalenes, dimethyl benzenes, trimethyl benzenes, dimethyl naphthalenes, diethyl benzenes, triethyl benzenes, triethyl naphthalenes, dipropyl benzenes, methyl ethyl naphthalenes, methyl ethyl benzenes, etc. The alkyl substituent to the aromatic hydrocarbon feeds may be straight chain or branch chain and, most often, will have 1 to 15 carbon atoms. In the preferred practice of the present invention, the feeds to the process are monoor di-methyl substituted aromatic hydrocarbons. Included within this preferred practice of the present process are toluene, l-methyl naphthalene, 2-methyl naphthaleneortho-xylene, meta-xylene, para-xylene, and dimethyl naphthalenes.

The hydrogen donors which are used in the process of the present invention are aromatic hydrocarbons which have been at least partially hydrogenated. Generally, these having one or more of the nuclei partially or totally saturated. Several non-limiting examples of such compounds are tetralin, dihydronaphthalene, diand tetrahydroalkylnaphthalenes, 9,10-dihydrophenanthrene, tetrahydrophenanthrenes, octahydrophenanthrenes, dihydroanthracenes, tetrahydroanthracenes, octahydroanthracenes, tetrahydrophenylnaphthalenes, dihydrochrysenes, tetrahydrochrysenes, octahydrochrysenes, tetrahydropyrenes, octahydropyrenes, tetrahydrofiuorenthenes, octahydrofluorenthenes, and the like. A particularly useful group of these hydrogen donor compounds are the hydrophenanthrenes, particularly the dihydrophenanthrenes and tetrahydrophenanthrenes, and the hydroanthracenes such as dihydroanthracenes and tetrahydroanthracenes. The source of the hydrogen donor used in carrying out the process of the present invention is immaterial. These hydrogen donors may be obtained by separating hydrocarbon fractions to obtain the aromatics which have been at least partially hydrogenated or may be obtained by hydrogenating specific aromatic hydrocarbons by conventional hydrogenation means. The present invention is not however, to be limited to any particular source or method for obtaining the hydrogen donors.

The amount of the hydrogen donor introduced into the non-catalytic reaction zone in accordance with the process of the present invention generally will be Within the molar ratio of 0.1:1 to 10:1 to the alkyl aromatic compounds in the feed which is to be subjected to dealkylation. It is somewhat preferred, however, that the amount of hydrogen donor be such as to cause a molar ratio to the alkyl aromatic compounds to be dealkylated within the range of 0.5:1 to 5:1.

One of the most critical features of the process of the present invention, is the temperature at which the process is operated. As noted from Example I above, relatively slight differences in temperature result in a significant difference in result. Generally, temperatures below 1020' F. are not used since at lower temperatures, the conversion of the alkyl substituted aromatic hydrocarbons to dealkylated product falls below desirable levels. Temperatures above 1475 F. are usually unnecessary and at such higher temperatures, there is considerable risk of rupture of the aromatic nucleus. In the preferred practice of the present invention temperatures within the range of 1100 to 1300 C. are most often employed. The pressures at which the present process is operated may vary from as low as about p.s.i.g. to as high as approximately 3000 p.s.i.g. and higher. Preferably, however, the pressure at which the present process is operated will be within the range of 300 to 1000 p.s.i.g.

Residence time of the reactants within the dealkylation zone will vary depending upon temperature. At lower temperatures, longer residence times are required and conversely, at higher temperatures lower residence times are required. Generally, however, the residence time will be within the range of 0.1 second to 20 minutes. In the preferred practice of the present invention, residence time within the range of 1 to 100 seconds are most often used.

What is claimed is:

1. In a process for the dealkylation of alkyl substituted aromatic hydrocarbons in a non-catalytic thermal reaction zone, the improvement which comprises subjecting a feed stock containing the alkyl substituted aromatic hydrocarbons to a temperature of l100 to 1300 F. and a pressure of 100 to 3000 p.s.i.g. in the presence of a hydrogen donor, said hydrogen donor being a hydrophenanthrene or a hydroanthracene, in the mol ratio of hydrogen donor to alkyl substituted aromatic hydrocarbon in the range of 0.1:1 to 10:1, and in the absence of added molecular H or other hydrogen donor.

2. The process of claim 1 wherein the alkyl substituted aromatic hydrocarbon has from 1 to 15 carbon atoms in the alkyl su-bstituents.

3. The process of claim 2 wherein the alkyl substituted aromatic hydrocarbons are selected from the group consisting of the mono-methyl and di-methyl substituted aromatic hydrocarbons.

4. The process of claim 1 wherein the alkyl substituted aromatic hydrocarbons are selected from the group coning of toluene, l-methyl naphthalene, Z-methyl naphthalene, ortho-xylene, meta-xylene, para-Xylene, dimethyl naphthalenes, and combinations thereof.

5. The process of claim 1 wherein the hydrogen donor is 9-10-dihydrophenanthrene.

6. The process of claim 1 wherein the residence time of the alkyl substituted aromatic hydrocarbons within the reaction zone is within the range of 0.1 second to 20 minutes.

7. The process of claim 1 wherein the pressure within the reaction zone is within the range of 300 to 1000 p.s.i.g.

References Cited UNITED STATES PATENTS 2,381,522 8/1945 Stewart 196-50 2,929,775 3/ 1960 Aristofl et al 208133 3,102,151 8/1963 Haldeman et al. 260672 3,145,238 8/ 1964 Kestner 260672 3,177,262 4/1965 Calkins 260-672 3,193,595 7/1965 Kenton et al 260672 3,198,846 '8/1965 Kelso 260672 3,256,357 6/1966 Baumann et al. 260-672 3,284,527 11/1966 Gill et al. 260672 3,288,873 11/1966 Moll 260672 3,288,875 11/1966 Payne et al. 260672 3,296,323 1/1967 Myers et a1. 260672 OTHER REFERENCES Curran et al.: Mechanism of Hydrogen Transfer, ACS, Div. of Pet. Chem, Preprints 10(2), C130-C14'8 (March 1966).

15 DELBERT E. GANTZ, Primary Examiner G. E. SCI-I'MITKONS, Assistant Examiner US. Cl. X.'R. 

